Mercaptans are highly useful in very numerous fields, for example as flavourings, odorants for gases, chain transfer agents in polymerisation, starting materials for the pharmaceutical or cosmetic industry, for the synthesis of antioxidants, extreme-pressure or anti-wear additives for lubrication. These examples do not in any way limit the uses of the mercaptans known at present and which can be prepared by virtue of the process of the invention.
In particular, the first of the mercaptans, methyl mercaptan (CH3SH), is very industrially beneficial, in particular as starting material in the synthesis of methionine, an essential amino acid very widely used in animal feed. Methyl mercaptan is also a starting material very widely used for the synthesis of numerous other molecules.
Mercaptans may be synthesised by numerous methods such as the sulfhydration of alcohols, the catalytic or photochemical addition of hydrogen sulfide onto unsaturated organic compounds, the substitution of halides, epoxides or organic carbonates by means of hydrogen sulfide, and others.
In particular, methyl mercaptan is currently produced industrially on the tonne scale from methanol and hydrogen sulfide according to the reaction (1):CH3OH+H2S→CH3SH+H2O  (1)
These processes have the drawbacks of requiring methanol (CH3OH), of synthesising hydrogen sulfide (H2S, from hydrogen and sulfur for example, which also then requires the synthesis of hydrogen), and give rise to by-products of dimethyl ether (CH3OCH3), dimethyl sulfide (CH3SCH3) type, and products of cracking and water, which implies numerous steps for purification of the methyl mercaptan.
By way of examples, the description of processes based on these reactions will be found in patent applications such as WO2013092129, WO2008118925, WO2007028708, WO2006015668 and WO2004096760.
It may prove economically advantageous (to avoid methanol synthesis) to wish to produce methyl mercaptan from carbon monoxide, hydrogen and hydrogen sulfide, according to the following synthesis scheme (2):CO+2H2+H2S→CH3SH+H2O  (2)
However, these processes have the drawbacks of requiring synthesis gas (CO/H2) and therefore carrying out steam reforming of a source of hydrocarbons, having the correct proportions between CO and H2, hence being able to adjust the CO/H2 ratio with what is referred to as the “water-gas shift reaction” (CO+H2O→CO2+H2), and synthesising H2S.
These processes also generally lead to large proportions of CO2 as by-product, and also to methane, dimethyl sulfide and water. By way of example, descriptions of these processes will be found in patent applications such as US2010286448, US2010094059, US2008293974, US2007213564.
Yet other processes have been described, and combine different reactions such as:                Formation of CS2 and H2S from methane and sulfur (3):CH4+4S→CS2+2H2S  (3)        Hydrogenation of CS2 (4):CS2+3H2→CH3SH+H2S  (4)        
It is also possible to use the excess H2S from reactions (3) and (4) in the reaction with methanol (reaction 1) or the reaction with synthesis gas (reaction 2) to further give methyl mercaptan.
These processes obviously combine the drawbacks described for reactions (1) and (2) with the additional difficulty of having excess hydrogen to carry out reaction (4). Descriptions of these processes will be found in patent applications US2011015443, or, more specifically in relation to reaction (4), in application WO2010046607.
Application WO200196290 proposes a process for synthesising methyl mercaptan directly from methane and H2S with coproduction of hydrogen. This direct reaction between methane and H2S occurs by means of a pulsed plasma with corona discharge. Since this application does not describe any examples of synthesis, it may appear difficult to imagine a process for the large-scale industrial synthesis of methyl mercaptan with this technology. Moreover, this process requires the synthesis of H2S if the latter is not available.
For its part, patent application EP0649837 proposes a process for the synthesis of methyl mercaptan by catalytic hydrogenolysis, with transition metal sulfides, of dimethyl disulfide with hydrogen. Although this process is efficient, it requires relatively high temperatures of the order of 200° C. to obtain industrially advantageous levels of productivity.
Those skilled in the art also know that it is possible to prepare methyl mercaptan by acidification of an aqueous solution of sodium methyl mercaptide (CH3SNa). This method has the major drawback of producing large amounts of salts, such as sodium chloride or sodium sulfate, depending on whether hydrochloric acid or sulfuric acid is used. These saline aqueous solutions are often very difficult to treat and the traces of foul-smelling products which remain mean that this method cannot be readily envisaged on the industrial scale.
The processes for synthesising mercaptans higher than methyl mercaptan also have numerous drawbacks. Thus, the substitution of alcohols with hydrogen sulfide requires high temperatures, and often pressures, and leads to undesired by-products of olefin, ether and sulfide type.
The catalytic or photochemical addition of hydrogen sulfide onto unsaturated compounds often occurs under slightly milder conditions than above, but also leads to numerous by-products formed by isomerisation of the starting material, by non-regioselective addition or by double addition which gives sulfides. Finally, the substitution of halogenated derivatives gives rise to processes which generate large amounts of effluents and saline waste which are not easily reconcilable with industrial processes.